1. Field of the Invention
This invention relates generally to the measurement of tape-tension in a tape recording and/or reproducing apparatus, and more particularly is directed to an improved tape-tension measuring device which is interchangeable with a tape cassette for measuring the tape tension in a cassette-type magnetic tape recording and/or reproducing apparatus, such as, a cassette-type tape recorder for audio signals or a cassette-type VTR (Video Tape Recorder).
2. Description of the Prior Art
In a cassette-type magnetic tape recording and/or reproducing apparatus, a magnetic tape is drawn or unwound from a supply reel and directed by guide rollers or pins so as to move in a tape transport path prior to being rewound on a take-up reel. In such tape transport path between the guide rollers or pins, the tape is successively engaged by at least one magnetic head for recording or reproducing signals on the tape and, a rotated capstan and a pinch roller pressed toward the capstan with the tape therebetween so as to move or transport the tape. In the described tape transport path, the tension produced in the tape at the run thereof extending from the supply reel to the nip between the capstan and pinch roller is an important factor in achieving the stable or uniform transport of the tape. Accordingly, in designing or assembing the magnetic tape recording and/or reproducing apparatus, the tension in the described tape run is measured and adjusted to a predetermined value.
It will be apparent that the described tape tension is determined, at least in part, by the force with which the pinch roller is pressed against the capstan for engaging the magnetic tape therebetween. Therefore, in the conventional method of measuring the tape tension, a spring balance is employed for determining the force with which the pinch roller is pressed against the capstan, and the tape tension is estimated from the force measured by the spring balance. In other words, the tape tension is indirectly measured so that the described procedure is rather troublesome and also error prone. Moreover, the relationship between the tape tension and the force with which the pinch roller is pressed against the capstan tends to vary with aging of the pinch roller or contamination of its surface, for example, with oily substances. Where such relationship varies substantially, the actual tape tension cannot be estimated with any degree of accuracy even though the force urging the pinch roller against the capstan is accurately measured.
It has been known, for example, as specifically disclosed in U.S. Pat. No. 3,771,358, having a common assignee herewith, to provide a torque measuring device adapted to take the place of a tape cassette in a cassette-type magnetic tape recording and/or reproducing apparatus for directly indicating the torque applied to the take-up reel of a tape cassette during a recording or reproducing operation and/or the back tension generated in the tape intermediate its engagement by the capstan and pinch roller and the supply reel. Such torque measuring device generally comprises a casing substantially equivalent to the cassette housing to be interchanged therewith and having openings at locations along one side of the casing for receiving the capstan and pinch roller and the usual recording and reproducing head and erasing head of the recording and/or reproducing apparatus. The casing contains first and second reel assemblies including respective inner ring portions to be rotatably coupled with the take-up and supply reel shafts of the recording and/or reproducing apparatus, outer ring portions rotatable relative to the respective inner ring portions against the resistance of respective spiral springs and a test tape having portions wound on the outer ring portions of the first and second reel assemblies and an intermediate tape run therebetween which is guided along the previously-mentioned side of the casing. Therefore, when measuring torque or tape tension in the recording or reproducing mode of the apparatus, the intermediate tape run is engaged between the capstan and pinch roller and is driven thereby, while the take-up reel shaft tends to take-up or wind the tape on the outer ring portion of the first reel assembly and the supply reel shaft exerts a drag on the unwinding of the tape from outer ring portion of the second reel assembly. The torque applied to the outer ring portion of the first reel assembly from the take-up reel shaft is indicated as a function of the angle of relative turning of the ring portions of the first reel assembly, while the back tension in the tape run between the nip of the capstan and pinch roller and the second reel assembly, and which results from the drag exerted by the supply reel shaft, is indicated as a function of the angle of relative turning of the inner and outer ring portions of the second reel assembly.
A number of problems are associated with the operation of the above-described torque measuring device. First of all, if the recording or reproducing mode of the apparatus is maintained for a substantial period of time to permit reading of the torque and back tension indications, then the test tape must be of substantial length so as to permit the winding and unwinding of the test tape on the first and second reel assemblies, respectively, during such period. The foregoing problem can be avoided, for example, as disclosed in Japanese Utility Model Unexamined Publication No. 4305/76, filed by the assignee of the present application, and which was published Jan. 13, 1976, by providing a slippable friction clutch in place of the spiral spring connected between the inner and outer ring portions of the first reel assembly associated with the take-up reel shaft. Such slippable friction clutch may be constituted by an elongated spring of square or other polygonal shape so as to have a plurality of sides and corners therebetween with the sides and corners of the spring being respectively pressed against the inner and outer ring portions of the first reel assembly. Further, the inner ring portion of the second reel assembly to be associated with the supply reel shaft is fixed relative to the casing. With such arrangement disclosed in the identified Utility Model Application, the outer ring portion of the second reel assembly turns relative to the fixed respective inner ring portion until the force exerted by the spiral spring resisting such relative turning is equal to the drive force that can be exerted on the test tape by engagement of the capstan and pinch roller therewith. When the spring force resisting further relative turning becomes equal to the drive force, the capstan and pinch roller slip relative to the rest tape and, simultaneously, slippage occurs in the slippable friction clutch between the inner and outer ring portions of the first ring assembly. Thus, unwinding and winding of the test tape occurs only to the extent necessary for the relative angular movement of the inner and outer ring portions of the second reel assembly by which the tape tension is indicated. Accordingly, a relatively short test tape can be employed with such test tape being automatically returned to its initial or starting position each time the tape-tension measuring device is removed from the recording and/or reproducing apparatus.
However, the above-described device from the identified Utility Model Application is still burdened by certain problems that are also inherent in the arrangement disclosed in U.S. Pat. No. 3,771,358. More particularly, in both cases, the test tape is engaged by at least the magnetic recording and reproducing head in addition to the capstan and pinch roller when measuring the tape tension in the recording and reproducing mode of operation. Such engagement with the recording and reproducing head adversely influences the indication of tape tension resulting from the driving of the tape by the capstan and pinch roller. Furthermore, the measurement of tape tension assumes a uniform or standard coefficient of friction at the engagement of the capstan and pinch roller with the test tape. However, through repeated use of the tape-tension measuring device, oily or other contaminating substances may accumulate on the test tape and thereby adversely influence the accuracy of the tape-tension measurements by varying the coefficient of friction at the engagement of the capstan and pinch roller with the test tape.